Patent GB-564774-A (GB)
Title: Improvements in or relating to spot welding machines
IPC: B23K11/31
Filed: 1943-05-12
Abstract: 564,774. Electric spot welding. SINCLAIR, J. M., and YOUNG, W. F. May 12, 1943, Nos. 7589/43 and 5447/44. [Class 83 (iv)] In or for a pedal-operated welding machine of the kind in which the movable electrode is actuated directly by the pressure of the operator's foot on the pedal, apparatus is provided for automatically maintaining the pressure of the electrode on the work independently of the pressure applied by the operator for a predetermined time after the initiation of the welding operation. The weld may thus be automatically consolidated or forged for the requisite time according to the thickness of the parts. The electrodes 30 are carried respectively by a fixed arm 12 and a movable arm 14 pivoted at 16 and having an extension 18 with a pivot 20 through which passes a back shaft 22 pivoted to the pedal lever 26, a spring 32 being interposed between an adjustable nut 34 on the shaft 22 and the end of the extension 18. The upper end of the shaft 22 carries a cross-piece 38 which normally keeps open a spring-closed switch 42. The shaft 22, or the pedal or electrode arm 14, is provided with a sleeve 60, Fig. 4, having one or more teeth 64, one of which is engaged by a pawl 66 on a solenoid-operated lever 68, to keep the shaft 22 raised for a predetermined time after the pedal has been pressed to bring the electrodes into contact with the work. The lever 68 is pivoted at 70 and one arm is connected through a spring 78 to a link 76 secured to the armature of the solenoid 74 so as normally to hold the pawl out of action. When the electrodes are pressed on to the work and switch 42, Fig. 3, closed by pressure on the pedal, a winding 48 is energized so as to close the welding contactors 50, the current passing through a normally closed switch 52. The current at the same time energizes a winding 56 but the opening of the switch 52 is retarded by a dashpot 54 so that the current passes for the desired time. Current also passes through a winding 82 in parallel with the winding 48 so as to close a switch 80 in a circuit containing the solenoid 74 so as to hold the shaft 22 in raised position. When the welding current is eventually cut off by opening of the switch 52, current through the winding 82 is also cut off but opening of the switch 80 is retarded by an adjustable dashpot 84 so that the parts are maintained under pressure for the requisite time independently of the operator. A signal lamp 86 in a circuit in parallel with the winding 74 is extinguished at the end of the forging period when the switch 80 opens and the pedal should be held pressed until the lamp is extinguished otherwise it is necessary for it to be slightly depressed at the end of the welding operation in order to release the pawl from the rack. The movement of the horizontal arm of the lever 68 is limited in one direction by the head of a bolt 88 and in the other direction by a nut 90. In another form, Fig. 6, the shaft 22 is held raised by a pawl lever 96 moved forward by a spring 98 and retracted by a solenoid 100 through a link 104. In this case the shaft 22 is held raised by the spring- operated pawl when the pedal is pressed and the welding current is connected as in the previous case. In this case, however, the winding 110, Fig. 7, in parallel with the winding 48, when energized opens a normally closed switch 108. When the welding current is cut off the winding 110 is de-energized but the switch 108 is provided with a dashpot 112 so that this switch 108 does not close again to energize the solenoid 100 until the weld is forged. The rack sleeve 60 also closes a switch 114 in the circuit of the solenoid prior to reaching its raised position and opens the switch again prior to reaching its lowest position so that the solenoid is de-energized when the machine is idle. When welding thin sheet metal the dashpots 84 and 112 may be omitted as the natural time lag of the relay switch is sufficient to provide for the consolidation of the metal. The device may be fitted to an existing machine.